The present invention relates to a method of synthesizing diamond. In particular, the present invention relates to a method of synthesizing diamond crystals and diamond films using hot-filament chemical vapor deposition.
Diamond synthesized by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) has many unique and outstanding properties that make it an ideal material a broad range of scientific and technological applications. See Y. Tzeng et al., Application of Diamond, Elsevier Publishers, 1991. A number of methods for diamond CVD are reported which utilize various gas mixtures and energy sources for dissociating the gas mixture. See P. K. Bachmann et al., Diamond and Related Materials 1, 1, (1991). Such methods include the use of high temperature electrons in various kinds of plasma, high solid surfaces on hot filaments, and high temperature gases in combustion flames to dissociate molecules such as hydrogen, oxygen, halogen, hydrocarbon, and other carbon containing gases. Typically, a diamond crystal or film is grown on a substrate, which is usually maintained at a temperature much lower than that of electrons in the plasma, the heated surface of a hot filament, or the combustion flame. As a result, a super equilibrium of atomic hydrogen is developed near the diamond growing surface of the substrate.
Atomic hydrogen is believed to be crucial in the diamond CVD process. It is theorized that atomic hydrogen is effective in stabilizing the diamond growing surface and promoting diamond growth at a CVD temperature and pressure that otherwise thermodynamically favors graphite growth. Consistently, the reported diamond CVD processes involve the use of hydrogen gas or hydrogen containing molecules. The most typical diamond CVD process utilizes a precursor consisting of methane gas diluted by 94-99% hydrogen. With these CVD processes, the super equilibrium of atomic hydrogen can be achieved at a varied percentage of molecular hydrogen in the gas mixture. However, these CVD processes depend on the effectiveness of the dissociation process in generating atomic hydrogen.
Chein et al., Proceedings of the 6th International Conference on New Diamond Science and Technology (1998), report using a high power density microwave plasma to deposit diamond in a precursor consisting of a mixture of methane and hydrogen with less than 50% hydrogen. Growth of diamond from oxy-acetylene flames utilizes a precursor consisting of acetylene and oxygen with a ratio of acetylene to oxygen slightly greater than 1 without additional molecular hydrogen being added. Diamond is deposited in the reducing xe2x80x9cinner flamexe2x80x9d where atomic hydrogen is a burn product produced by the high temperature flame. In addition to atomic hydrogen, there are plenty of OH radicals present near the diamond growing surface inside the flame.
OH and O radicals can play another role of atomic hydrogen in the diamond growth process. That is, preferential etching of non-diamond carbon, which results in a net deposition of high purity diamond. Small quantity of oxygen (0.5-2%) or water vapor ( less than 6%) added to the methane and hydrogen precursor is reported to improve diamond crystallinity and lower the diamond CVD temperature. See Saito et al., Journal of Materials Science, 23, 842 (1988), and Kawato et al., J. Applied Physics, 26, 1429 (1987). The quantity, whether small or large, of oxygen and water in a precursor or feedstock is a relative term depending on many other process parameters. Diamond has also been grown in a microwave plasma of a precursor consisting of an acetone/oxygen mixture with a molecular ratio near 1:1. See Chein et al., Proceedings of the 6th International Conference on New Diamond Science and Technology (1998).
Most of the diamond CVD processes involve the use of one or more compressed gases. Typically, such CVD processes utilize a compressed gas precursor consisting of 1 vol. % methane gas diluted by 99 vol. % hydrogen. These gases usually must be precisely controlled by electronic mass flow controllers to ensure the accurate composition in the gas precursor feed. In U.S. Pat. No. 5,480,686 to Rudder et al. (xe2x80x9cRudderxe2x80x9d), a method of diamond growth is disclosed that utilizes a radio frequency (xe2x80x9cRFxe2x80x9d) plasma in a precursor consisting of a mixture of water (more than 40%) and alcohol. No compressed gases are needed for this diamond CVD process. However, water has a low vapor pressure at room temperature, and condensation of water in the cooler part of the reactor manifold may be a concern. Also, water has a high freezing temperature making it easy to freeze at the orifice of a flow controller where liquid vaporizes and enters a low pressure reactor chamber. Further, comparative example 6 of Rudder shows that the hot-filament assisted CVD process using 18 standard cubic cm (xe2x80x9csccmxe2x80x9d) water and 40 sccm ethanol at 80 Torr at a filament temperature of 2100xc2x0 C. located about 2 mm from a silicon substrate at 650xc2x0 C. deposits only fine grain graphite. In contrast Japanese Patent, Kokai Sho 6211987-18006 to Komaki et al. reports the deposition of diamond. This indicates that hot filament can not be used successfully for growing diamond in an atmosphere of high oxygen or water content without special cautions. In a highly oxidizing environment, hot tungsten filaments are simply burned off quickly.
Nevertheless, the hot filament assisted CVD method is desirable because diamond films can be deposited on large-area and/or irregularly shaped objects using inexpensive equipment. Thus, there remains a need for an economic method of synthesizing diamond utilizing hot filament CVD. Further, there remains a need for a method of diamond CVD which employs the benefits of radicals that preferentially etch non-diamond deposits at low substrate temperatures to provide the deposition of high quality diamond. It is to the provision of a method of hot-filament assisted CVD of diamond that meets these needs that the present invention is primarily directed.
Briefly described, the present invention relates to a method of synthesizing diamond that enables the economic growth of high quality diamond crystals and diamond films using a liquid solution as the feedstock and a hot filament as the means of dissociating and reacting the vapor of the solution. In the method of the present invention, a precursor comprising methanol and at least one carbon containing compound having a carbon to oxygen ratio greater than one is fed into a deposition chamber through a liquid flow controller such as a needle valve. The precursor is a solution comprising 90-99% of methanol and 1-10% of at least one carbon containing compound such as ethanol, isopropanol, and acetone. The precursor is pre-mixed prior to entering the deposition chamber. The solution vaporizes as it enters the low pressure deposition chamber. The vaporized precursor comprises the same composition as the solution. When the vaporized precursor passes a hot filament heated to above 2,000xc2x0 C., it is dissociated to generate OH, H, O, CH3 and other molecules and radicals.
The substrate generally is sheet or wafer of silicon, copper, aluminum and molybdenum. Additionally, the substrates can be polished using 1 xcexcm diamond paste prior to the deposition process. Typically, the substrate is mounted at a distance, for example, 2 cm, from the hot filament to insure that a desired substrate temperature and composition of radicals transported from near the hot filament are achieved. A spiral tungsten wire of 1.5 mm diameter is passed with 110A 60 Hz electrical current. The reactor chamber pressure is generally maintained at 40 Torr. The silicon substrate of about 25 cm by 2.5 cm is heated by the hot filament to about 600-1000xc2x0 C. as measured by a dual color optical pyrometer. Diamond is grown at a rate of about 1 xcexcm per hour when 4 grams ethanol, 2.5 grams isopropanol, or 2.5 grams acetone is mixed with 100 grams methanol and used as the precursor.
Various other objects, features and advantages of the present invention will become known to those skilled in the art upon reading the following specification when taken in conjunction with the accompanying drawings.